Posted
by
Soulskill
on Sunday November 15, 2009 @11:21AM
from the dream-big dept.

An anonymous reader writes "Robert X. Cringely once again educates and amuses with his take on how we could clean up the garbage that's in orbit around Earth. I cannot vouch for his math, but it makes sense to me. Quoting: 'We’d start in a high orbit, above the space junk, because we could trade that altitude for speed as needed, simply by flying lower, trading potential energy for kinetic. Dragging the net behind a little unmanned spacecraft, my idea would be to go past each piece of junk in such a way that it not only lodges permanently in the net, but that doing so adds kinetic energy (hitting at shallow angles to essentially tack like a sailboat off the debris). But wait, there’s more! You not only have to try to get energy from each encounter, it helps if — like in a game of billiards or pool — each encounter results in an effective ricochet sending the net in the proper trajectory for its next encounter. Rinse and repeat 18,000 times.'"

Mod parent up. In addition, tiny specks acting as micrometeorites are probably a much bigger problem than the bigger avoidable pieces. Hitting all that big junk together in a net at orbital speeds will probably result in even more micrometeorites.

This also leads me to think that you'd need less of a 'net' and more of a 'sheet'. One would then wonder, depending on altitude and sheet size, when atmospheric drag becomes an issue.

I remember an SF story over a decade ago where the author (maybe Pournelle?) had a similar sweeper idea and they used aerogels [nasa.gov] to scrub Earth orbit. I say let's call it a trap instead of a net or a sheet, since that describes its function as opposed to its form. That said, I don't think the orbital mechanics would work the way Cringely thought it might. Higher orbits go slower, so the hanging sheet/net (moving at the speed of the orbit of the center of mass of the sheet/tug) is going to be accelerated by impacts from the space junk which will lead the whole thing into a more elliptical orbit, not a shallower circular one. That's not a good way to arrange a slow spiral down to clean up orbits gradually. And that doesn't even begin to consider the stuff in eccentric orbits. So Cringely's idea isn't original, and if he read the same story I did and is subconsciously re-iterating the idea, he's not even getting it right. Now, if you balanced your trap with a larger solar sail and used it to keep your orbit more circular, you might have something.

As for atmospheric drag, if it's an issue for the trap, it will be an issue for anything flying that low. The space junk flying low enough for atmospheric drag to be a factor is a self-correcting problem.

Hmm, interesting point, and idea. Though I imagine in RL the cost of that much aerogel would be prohibitive, at least at this point.

I think the self-correcting factor comes in 10x, 1000x, etc. flavors, depending on velocity/mass/coefficient/altitude. I think 100,000 years is the figure I heard at one point (no source, sorry) for non-LEO objects. So while your correct that waiting a few decades corrects the LEO problem (or perhaps a few centuries) for nearly anything, the HEO objects would, I imagine, be

It won't work... there are too many unknowns and no way to fix the scow when (read inevitable) things go wrong.
The scow approach can really only be designed upfront and then implemented after the fact.... risky.
The lasers idea that he dismissed out of hand early on in the article actually makes more sense. Except that the lasers
aren't intended to vaporize the entire object, but a tiny fraction to induce a deceleration so that the orbit can decay faster.
The laser approach can go through spiral devel

Wouldn't it be bad catching all of the space debris in a giant net, when the net itself will eventually come back down to earth. Individual space junk coming out of orbit isn't as bad since it's not all falling in the same place and it's small enough to mostly burn up in the atmosphere, but if you've got this huge net there's a lot more junk to burn up with a much more localized crash site.

Plus this thing bouncing around like a billiard ball seems likely to catch something that isn't junk...

The risk of snagging one of the numerous live satellites would certainly be a problem. Re-entry, though, could be handled by picking an unloved chunk of ocean(hardly a limited resource) and just aiming for that.

The problem with space junk is that there's thousands of piece of it flying around that can damage spacecraft, re-entry isn't really the problem. That's actually preferable to losing a few of your spacecraft to loose pieces of material in orbit.

Who says it needs to re-enter? If the bits of junk are all lodged in a larger net structure which behaves in a predictable manner, it could just be left up there as a sort of orbital junkyard. The proposed designs for a space elevator require a chunk of ballast to keep the tether taught... Why not a bunch of discarded booster shells and such, tacked together? It took a lot of energy to get that stuff up there... Why waste it?

I'd mod you up if I had points. Apparently Cringely hasn't thought about how valuable a few hundred metric tons of refined materials would be in orbit. Instead he says "Nope, we have to gather the stuff and bring it back to Earth." He fails to realize that _someone_ would certainly pay for access to all of that material. He also fails to realize that a polar orbit intersecting an equatorial orbit will result in a relative velocity of about 10 kilometers per second, which equates to 50 megajoules per kilogram. Carbon nanotubes or not, nothing is going to withstand such a large amount of energy in such a small area, repeatedly, along with whatever centripetal forces are acquired from off-center hits from debris.

A visionary he might be, but a practical engineer he is definitely not.

50 megajoules per kilogram is only a problem if the net is not affected by the impact. Have the net take on some of the momentum of the impact and you can absorb a lot of the energy. You can also use an elyptical orbit to decreases the relative velocity.

At speeds above Mach 8.0, you can drive a pencil through a 100mm armor steel plate - even the pencil tip stays intact and sharp.

At 36,000km/s (equal to Mach 36 at sea-level), the net or carbon fiber construction will not even have a chance to absorb anything. The net itself might be able to absorb this momentum and energy level at a whole, but I seriously believe a metal piece will just blast right through it, instantly shearing the filament at molecular level. The inertia of a single carbon nanotube will probably be all that is needed to cleanly cut it off.

At speeds above Mach 8.0, you can drive a pencil through a 100mm armor steel plate - even the pencil tip stays intact and sharp.

At 36,000km/s (equal to Mach 36 at sea-level), the net or carbon fiber construction will not even have a chance to absorb anything. The net itself might be able to absorb this momentum and energy level at a whole, but I seriously believe a metal piece will just blast right through it, instantly shearing the filament at molecular level. The inertia of a single carbon nanotube will probably be all that is needed to cleanly cut it off.

But if it causes these small pieces of metal to fall towards the Earth, where the atmosphere will do a lot of the work for us, isn't that good enough?

I'd say "away from the Earth" too, but I think I'd rather not like to lose a large amount of metal to the interplanetary void...

At speeds above Mach 8.0, you can drive a pencil through a 100mm armor steel plate - even the pencil tip stays intact and sharp.

Though I completely agree with your overall point, I'm curious if you have a citation for this sentence. The plate and pencil are in relative motion, yet apparently the impact drills a hole through the plate without even dulling the pencil? I tried googling for an experiment like this with no luck. Now I'm just trying to figure out what insane combination of high-speed photography and a hypersonic wind tunnel with a "pencil of death" feature would be required for proof...

The contents that would be accumulated wouldn't be compressed into a 'hard' meteor as it re-entered the atmosphere. After a few seconds it would break apart into numerous pieces with a huge surface area. With slight considerations to have it re-enter over an ocean, the risk from these items would be very very low.

I thought they were just in the early stages of establishing a ring-world, in terrestrial orbit. Oh well...

There will of course, be no such mission, headed by NASA, or any other fraction of the Federal United States. That banana republic operates on such a scale, only when there is substantial room for contractor and supplier rip-off. If Cringeley can figure a way for DynaCor to pocket a billion on the side, instead of increasing fuel efficiency in spaceflight? It'd happen next year.

Perhaps Cringely doesn't have a clear idea what sort of debris we are dealing with here

There are, certainly, some big chunks out there; but unpleasant enough(and far more numerous) are the little flecks of paint, bolts, and general fragments of this and that zipping around at bulletesque velocities.

Either this "net" will be made of very close-woven unobtanium, of the sort that we don't yet have, despite decades of interest in the personnel armor industry, or it will have to be a vast spongy particle trap, of the sort whose volume would be completely prohibitive for any available launch mechanism.

Not only that, but does he realize how LARGE that space is? Can you imagine saying to somebody, take your yacht, and sail around the oceans picking up 18000 pieces that go around with vastly different speeds (and orbits)? Now do this in 3D instead.

Moreover, the delta v's involved are probably quite a lot larger than one would expect.

And as you say, the big pieces are tracked and show up on radar, it is the little pieces that hit unexpectedly.

As long as you're traveling at the same speed and direction as the bulletlike flecks, you don't have to worry about damage. Given that space is a frictionless environment, it's actually fairly easy to accomplish this. We do it every time we dock with the ISS.

Cringley seems to be suggesting traveling slightly slower, as to absorb some kinetic energy in the impact, while preserving the integrity of the net. This sounds pretty cool in theory, although there are a few problems in practice, such as tracking all the tiny bits of debris, having enough fuel to maneuver, and ensuring that you don't get caught between two pieces of junk traveling in opposite directions.

It's a difficult problem to be sure, but I wouldn't write it off entirely.

As an alternate proposal, would it make sense to put huge blocks of aerogel (or a similar substance) into orbit? Junk that strikes the blocks would either get caught inside, or pass straight through (but lose some kinetic energy in the process, leading to its gradual orbital decay or capture). Aerogel itself has a low enough density that loose chunks of it would be relatively harmless to passing spacecraft.

"As long as you're traveling at the same speed and direction as the bulletlike flecks, you don't have to worry about damage"

That's not true. Consider two objects in the same circular orbit traveling in opposite directions exactly 180 degrees apart. They are traveling in the same speed and direction (in physics that's called velocity) and will surely collide.

By same orbit I meant that their angular momenta would have opposite signs.

Think of the two objects, satellites, having telescopes that point straight ahead, that is, the line of sight is tangent to the orbit, and they both are focused on some distant star and are moving toward that star at that instant. At that instant they have the same velocity.

while your statement is true, its also mildly retarded. yes, 2 objects on the opposite sides of a circle, traveling in opposite directions around the circle, for a short moment are traveling the same direction relative to one another, *but thats not what we're talking about here* we're talking about 2 objects traveling the same direction around the orbit.

your argument, while a marvelous display of spatial relationships, is pointless in relation to the actual proposal at hand.

Assume a block of aerogel 10 meters on a side - so a frontal area of 100 m^2. That's pretty big, and it won't get any bigger unless we figure out how to manufacture the gel in space:
Agel = 100m^2
= 0.0001km^2

Velocity in leo is around 7.5km/second, relative to the ground.
Vgel = 7.5km/s

Let's assume that we are just trying to sweep the entire volume of space once, ignoring that things are moving etc. Even one sweep of the volume would certainly clean up a lot, if the orbit of the gel is tangent to the orbit of most of the junk. So we just pretend that the block of gel is flying down a tunnel, basically - frontal area times velocity * time equals volume cleaned:

Every velocity in space in orbit is super-bulletesque. It's the relative velocities that matter. I could catch the paint fleks with any old material, if, the relative velocities were reasonably close. Indeed, if you launched me out of a cannon next to a bullet fired out of a rifle, I'd almost be able to catch the bullet with no harm to myself. It's just the launching and the landing that would suck.

Perhaps Cringely doesn't have a clear idea what sort of debris we are dealing with here.

No, he doesn't seem to have a clear idea of what debris is, or what orbital energy is, or how orbits work, or how BIG space is.

It is reasonable to clear debris up from Earth orbit... but not the way he proposes.

I'm afraid I have to agree with the people saying that this is not a workable idea. He needs to put some numbers to it. He's going to catch basketball sized objects in a net? Have he thought about what happens when a massive object hits something at several miles per second? I'd say, picture tr

I would think that if we went 'spongy', it would make sense to send up a few cubic meters of a densely packed raw material and then extrude it in space; after all, the whole point of those materials is how low density they are. One jar of popcorn kernels is easier to transport before they're popped.

Do popcorn kernels pop when exposed to vacuum? I may have solved the issue, right there.

But I think that I would prefer a set of these, and dispense of them after a shorter time (burn it up or capture it for material studies). For starters, imagine accumulating a bunch of that junk together and then losing the ship. It could actually make things worst.

Also, this would be a good use for the tug concept. At some point, a tug will be useful for space. This could help push the concept.

IF (and I know it is a big IF) it were possible to "manufacture" aerogels in space, this material could be ideal for capturing/de-orbiting small pieces of debris that would be too difficult/expensive to chase and capture the traditional way (via space tug or whatnot) but still poses a threat. Aerogels have already proven themselves as capable of capturing extremely fast (although tiny) particles moving at literally astronomical speeds without itself disintegrating. It was used precisely for this reason in both the "Stardust" and "Genesis" probes.

Now imagine instead of the small plates that were on these probes a very large slab tens or hundreds (thousands?) of meters on a side that would, over time, slowly intercept the smaller particles. Larger fragments would still go right through but might lose enough kinetic energy (without fragmenting and making the problem worse) so as to de-orbit themselves. The only thing that might make this remotely possible is the thought that the aerogel is so light (lighter than air) that a really huge piece could be put into orbit without spending billions in launch something heavy. Of course the only way to keep the launch volume reasonable is to MAKE it in space. Once in space, an ion engine would be required to counteract the atmospheric drag (and loss of kinetic energy from the impacts of the space debris).

By "manufacture" I mean the raw material (I guess it some sort of silicate compound) would have to be brought up from earth but since the resulting aerogel is 99.9% empty space, a little could go a long way. I understand that one way to produce it requires a super-critical liquid carbon-dioxide solution; obviously the CO2 would have to be recycled or better yet would be if a means of producing it directly in vacuum. Chemists, any ideas?

Aerogel is pretty brittle, though. One can imagine flaking as a result of impact and no real net benefit debris-wise. I think something ductile would be a better choice. Like, say, steel foil, perhaps.

You want every impact to slow down the object enough that de-orbit occurs within months instead of years, and any detached pieces of the cloak should also have a similar profile. Only the intact cloak itself should be able to orbit for a time, until it is decided to de-orbit it, as well.

Didn't know that about Aerogels. How about wrapping the whole thing in a (very) thin layer of "saran" wrap? (to non-Americans that's transparent plastic wrap). It'll keep the whole thing from fragmenting without (hopefully) adding too much to the weight.

Also, I'm hoping that little pieces of the Aerogel will be relatively "harmless" upon impact (is anything harmless at 25,000 mph)? Perhaps the "wrap" could be made non-transparent to something like UV while the Aerogel could be tailored to disintegrate up

Aerogel [wikipedia.org] has been shown to be able to pick up even the smallest flecks of material for the Stardust [wikipedia.org] project.

Since it's the smallest things that are the trickiest (huge bits are easily tracked), we need something that will not only absorb the energy of the impact, but also keep the debris in place. Thus, Aerogel is a good fit.

And orbital speeds are high, sure, but the slab is also moving at orbital speeds, which will reduce the impact speed significantly as well. And while it may not stop a bolt or bigger item on its own, there is nothing stopping you from placing thick solid metal back-stoppers every after few meters of Aerogel.

Then you end up with a gelatinous cube effect with the stopping power of several meters of solid steel.

In a sense, yes. From what I understand the problem with space debris are the small size and the high kinetic energy. If you've got everything in a big lump it should be easier to de-orbit stuff and get it to burn up in the atmosphere.

It makes sense to capture and lose the small pieces. BUT, the large ones are lots of material in space that took a lot of fuel to get there. That would be a shame to lose those if they are together. Seems like we can push those into a higher orbit out of the way and then use them in the future.

It's not quite that simple. If the orbit is elliptical then two orbits can intercept even though they have different energy level (average heights). If the two objects in the two orbits join then the one in the higher orbit will lose energy and the one in the lower orbit will gain energy (which corresponds to average height). The resulting object will have the same momentum as the vector sum of the momentum of the two objects, which will give it a new orbit. If you start in a low and highly eccentric orbit, after a number of such collisions you may end up in high and relatively circular orbit (or not, depending on the collisions). Cringely is broadly right that it is probably possible to design an orbit such that the net eventually collects everything. Unfortunately, 'eventually' in this case can mean several million years, possibly longer.

...means the net will lose speed every time it captures some junk. The author needs to take high school physics again.

You might want to consider a good orbital mechanics course yourself. If, as an example, I am travelling in an elliptical orbit with apogee at, say, 500 km and perigee at 300km altitude, and I hit a bolt in a circular orbit at 500 km, then I have just run into something that is going FASTER than me.

Which means that I'll speed up slightly, raising the perigee of my orbit.

You don't need to hit it head-on, and you don't need to bleed all of the orbital energy to put the debris chunk into a rapidly decaying orbit.

Presuming that there is only a countable number of objects (a few thousand, tens of thousand, even a million), it's much more reasonable to approach "just enough" to knock them into a month-long freefall instead of a decades long, one at a time. A computational nightmare, perhaps, and definitely extremely tricky to line up the right piece of debris without putting yo

To drop from a higher altitude to a lower altitude you have to lose kinetic energy, not gain it. Furthermore, everything is not traveling in the same direction. There are many different orbits and junk is in all sorts of them. So some junk you'll never "net" since it's traveling in the same direction as the dejunker, and other junk is traveling exactly opposite and will slam into the net with twice the velocity of the denetter's current orbital velocity. Furthermore if the junk's orbit is 90 degrees to the dejunker, it will never be caught either. Even if the orbital paths crossed, it would probably just destroy or damage the dejunker satellite (paint fleck or rachet wrench).

So it wouldn't seem that his idea stands the common sense test (or physics for that matter). But this is just slashdot and I am not an orbital-mechanics expert. I failed that class at the starfleet academy (or was that temporal mechanics).

Orbits are a combination of potential and kinetic energy. Depending on what orbit you are in, and what phase of the orbit you are in at the time those numbers will flip around being the predominant one.

So, depending on what you are doing, where you are now, and where you want to go you may have to do some counter-intuitive moves.

Bigelow has a small line to build Kevlar material for their space stations. It is suppose to handle 17000 mph pieces. Seems to me that BA might want to get in on this if funds were to be done. At the very least, it helps gets their line started with building items.

cringely sounds more and more like a clever junior high school student. nothing wrong with that, if you're in 8th grade. but i mean seriously, the volume of 3-d orbital space determines among other things the energy and time required to sweep it "clean." be almost faster just to wait for the junk to re-enter. cheaper and cleaner certainly.

Why a net? There's no resistance in space and no medium that needs to pass through the net!
Make it a big metal cup like the back of a dump truck. Drawback would be increased payload for launch, but it helps remove a lot of the risks others are describing such as the net breaking or contents of the net breaking up into smaller pieces.

He does need to apply a little math there. Dragging a net behind might happen due to (weak) atmospheric friction, but you're not going to sneak up behind debris and gently catch it in the net. Something in a lower orbit is moving faster relative to you, so the debris will come from 'behind'.

Also, you're going to be waving a huge net around and hope to only get close enough to things that are traveling at speeds which are only slightly different. Somehow you have to not catch working equipment, and not

"It won't always be possible, of course, to gain energy from each encounter, but that's why we start in a higher orbit, so as energy is inevitably lost it can be replenished by moving to a lower orbit."

Changing to a lower orbit will increase velocity, yes, so in a sense you're trading off potential and kinetic energy... but in the sense that matters... maintaining the ability to change your orbit... it doesn't matter if you go up or down, it matters only that you are changing your orbit. Any change in orbit

Consider for a moment that the sphere of a high orbit is larger than the size of the earth. Then consider all the orbital altitudes (like layers of an onion) which need to be "scoured", and you're talking about an amount of space that is many times the total surface area of Earth.

That's a whole lot of territory to cover, even for a large army of scour-ers.

I say we just send Superman up there with a big net and let him make a bunch of trips around the planet to clean everything up. Then we don't have to worry about conserving the energy in the pieces or any of that crap, because Superman has energy to spare.

Not only is this faster and safer, but much more entertaining. I think they could actually make a profit on this, when they figure in the comic book sales.

The notion of capturing energy from objects already in orbit is intriguing - but I doubt that "tacking" is sufficient to explain how this works. Tacking occurs when two fluids are connected by airfoils; moreover, the essence of tacking requires the deflection, or bouncing, of the fluids - not the collection of same.

So, in what way could you approach an object and steal its energy. But before that question, what does it mean to steal energy from these captured items? If the trash ends up a part of the garbage scow's orbital dynamics, then "stealing" energy is moot - unless the trash is ejected into a less energetic orbit, the scow cannot end up with a more energetic orbit - which of course defined the solution. The desired "net" may be an electromagnet on a long wire. The intercept is made with a near miss, such that trash and the scow end up like a double-star, tumbling around a common axis - then the electromagnet is released in a moment when the trash is tumbling counter-orbital, leaving the trash in an inferior (and hopefully terminal) orbit - and the scow in a new trajectory of choice - based largely on the intercept angle (to establish the tumble plane) and the release timing to select the angular acceleration.

You could try reading the summary next time. His proposal was for one flight, not 18000. I imagine his plan is still impractical for lots of reasons (you probably can't get enough impulse from each piece to approach the next one at a low enough speed, etc.), but it's still not as bad as your suggestion of 18000 manned space flights.

You must be new here. We don't read the summaries, let alone the friendly articles. Hell, any day now I expect most/. readers will stop reading the headlines, too, and every article will be a homogeneous mishmash of vim vs. emacs arguments, libertarian propaganda, and goatse links.

A better idea might be to use the concept of induction to our advantage. Create a satellite that creates a several kilometer diameter magnetic field bubble and fly it through the debris at high velocity. THe debris is most likely conductive and would have a current induced in it causing a drag force against the janitorial satellite. The orbits that cause the most drag are ones that run counter to the craft so they'll probably be nudged into a lower orbit by the drag. The janitorial satellite will use solar power and a space tether to stay in its current orbit. Any satellites that need to stay up there and aren't considered debris can be tracked much more easily and you could just shut the EM field down upon close encounter with them.The craft would use very little propellant and would probably work better than a net anyway. Just have a few craft like these flying around and acting like an immune system that kills off targets that are a danger to other craft.

That's not a bad idea. The real questions would be how much of a drag force could you create at a given distance? The junk is distributed in a cloud around the planet so encounters with junk could be hundreds of meters? Kilometers? Getting closer would require propellant. The field strength is limited by the amount of power you can generate, which ain't much from solar cells. The end effect is it may be completely infeasible because of scale. I wouldn't know how to work the numbers, but maybe someon

It depends on how much debris you are deorbiting and how large the area you are using for solar power generation. If power is a problem, supplement the power generation with a nuclear battery or two and every time they ae exhausted, swap them out for fresh ones.

As I had previously mentioned, the craft uses a technology otherwise known as Tether propulsion [wikipedia.org] which only uses electrical energy to provide a propulsive force against Earth's magnetic field. It works like an electric motor in a way.

That's assuming all (or even the majority) of the debris is ferrous. Titanium, aluminum, ceramic, propulsion slag, and leaked coolant are at least some of the debris that's up there, and it would happily float past a giant magnet like it wasn't even there. Lets not forget rock debris that's been floating around up there since before we (humanity) had a space program.

Don't forget, attraction and repulsion work both ways too. So the satellite pulls the object out of orbit a li

You may wish to refine your knowledge of inductance [wikipedia.org] which is not dependant on a material being ferromagnetic. Also, the craft is designed to sweep through about 800km^3 of space (10 km diameter bubble) every second and would be capable of cleaning a layer of space covering the whole planet 30 miles thick in a single year. More if the size of the bubble is increased.

I think that would be like trying to hit a bullet with another bullet and it's been tried with the US missile defense system with far slower moving objects and it was a complete disaster. Also, you are correct that the target would probably get broken into several pieces and pose a hazard until it burns up.

He's not proposing 18,000 spaceflights manned or otherwise. He's proposing a gigantic billiards shot where all the balls are in motion, salvaging the motion of some of the balls to line up the next one and eventually encounter and sink all the balls in one shot.

Then he's got some weird ideas about orbital energy this "net" concept that seems tricky (although a sufficiently strong, ductile net would increase the target area for intercept and it doesn't matter if the net gets torn to sh

How much fuel is this thing going to have to hold to make tens of thousands of adjustments.

How much sensitivity does it have to have to actually get close to this stuff?

And, what if it screws up and gets tagged really hard. Orbital velocities of any object can smash just about any other object. What unobtanium is this thing going to be made of? A net? Are you fucking kidding? The stuff that can kill satellites is smaller than a grain of sand. If by "net" he means "bigass wall" then m

I agree totally. He's an idiot. Most people who have read his stuff know that though.

There's way too much space to cover. I talked with some friends in the past about the same idea, but we realized it wouldn't work, without putting ourselves in front of the world to look like idiots.:)

I don't think putting water up would help much either. It may encourage some of the very low debris to deorbit sooner, but not the majority of the junk that's up there. The r

Only make some sort of sense if the summary had described the net as being in _front_ of the spacecraft, so that collisions increase the spacecraft's speed (which is in higher orbit and thus moving slower).

Even so, the net is more likely to cause stuff to break into smaller and harder to track pieces. And definitely bad if the net breaks...